This invention relates to communications sent through a power distribution network and, more particularly, to a point-to-point communications system by which information is readily transmitted from any one location within the power distribution network to any other location within the network. Specifically, the present invention relates to digitally demodulating TWACS® (Two Way Automatic Communication System) outbound signals including outbound preamble detection.
Power line communications systems are known in the art. A typical system enables a utility to send messages over its power line to or from a central location such as a substation to most, if not all, of its customers connected to that site. The messages involve such things as current electrical usage at the customer's site, polling requests to determine whether or not an outage has occurred within a service area, or commands to reduce or shut-off the amount of power provided to a load at the customer's site during periods of peak electrical usage. Replies received from the various locations to which messages are sent enable the utility to determine its current operational status, as well as changes that may need to be made to reconfigure the power distribution system for changes (or prospective changes) in its operating circumstances.
Electrical usage has grown significantly in recent years so that, over time, the demands placed on utilities have greatly increased and many utilities are now hard pressed to maintain adequate levels of service to their customers. Similarly, the demands placed on current communications systems employed by these utilities to support their operations have also greatly increased to the point where it has become difficult for these systems to timely provide the information necessary for the utility to operate at the level at which it needs to operate. For example, the amount of information required by the utility, on an almost continuous basis, has expanded to the point where the information throughput (data transmission rates) required of communications systems is at, or near the limits of the communications system's capabilities.
TWACS outbound signals have been decoded using comparators and timers. In this way the digital processor is supplied a sequence of times at which the incident signal has taken on known values. This has the advantage of not requiring expensive hardware.
The present analog outbound receiver requires a large signal-to-noise ratio (SNR) in order to maintain a reasonable bit error ratio. It is designed to work with a specific outbound signal constellation. This constellation contains only two signals, each with a duration of two complete mains cycles. Thus, the data rate is only 1 bit every 1/30 seconds, or 30 bps. This constellation size can be increased to as many as 16 symbols for this signal duration, so that up to 4 bits may be transmitted every 2 cycles giving a maximum data rate of 120 bps. However, adapting the present receiver to the new constellation would be difficult if not impossible.
There is a need for an outbound receiver using analog-to-digital conversion that could be adapted to the new constellation and which could be implemented on general purpose microprocessors.